Namespace fixup for D68340 build on MSVC

we seem to run into issues with nested namespace lookups in recently landed
D68340 so just make them explicit.

clang/lib/Driver/ToolChains/AIX.cpp

@@ -16,6 +16,8 @@
 
 namespace aix = clang::driver::tools::aix;
 using AIX = clang::driver::toolchains::AIX;
+using namespace clang::driver;
+using namespace clang::driver::tools;
 
 using namespace llvm::opt;